1. Field of the Invention
The present invention relates to apparatus for mixing and blending flows of thermoplastics material and the like, and, more particularly, to the use thereof in plastics extrusion and injection molding machinery.
2. Prior Art
It is important in many fluid flow applications that the constituents of a flow of viscous material be intermixed as the material is fed along a confined flow path. One such application which has long presented a challenge to those skilled in the art is the need to thoroughly mix semi-solid constituents of a flow of thermoplastics material as the material is being heated and melted in a plastics forming machine for delivery to an extrusion die or an injection mold.
In thermoplastics extrusion and injection molding machinery, it is customary to load solid pieces of plastics material, in granular or pellet form, into a hopper. The hopper, in turn, ducts this material into an elongate, barrel-like, auger-fed body for heating and blending as the material is fed under pressure along a confined flow path defined by the body. As the particles or pellets precess along the flow path, they begin to melt and take on a semi-solid form. If these semi-solid constituents are not uniformly melted, mixed and blended before they are fed through an extrusion die or into an injection mold, a plastics product lacking in homogeneity may result. Where plastics products having intricate, and/or thin cross-sectional portions are being formed, these products are particularly susceptible to malformation in the presence of almost any lack of homogeneity in the supply of production material.
A further problem which is often caused by incomplete mixing and blending of a supply of heated thermoplastics material is non-uniform coloring of the resulting plastics product. In the forming of colored plastics products, particles of colorant are introduced simultaneously with pellets of thermoplastics resin into the barrel-like body of an auger-fed plastics forming apparatus for concurrent melting and mixing. The colorant is selected to impart a desired color to the base resin, and when properly mixed therewith during heating, provides a homogeneous flow of thermoplastics material which may be utilized to form a plastics product of desired color. However, if the colorant is not thoroughly and uniformly mixed and blended with the base resin, the resulting plastics product will exhibit unwanted non-uniformities in color.
Non-uniform mixing of a flow of thermoplastics material can result, in part, from differences in the sizes of the particles which are being mixed and melted. Smaller particles tend to melt more readily than larger particles. If the larger particles are incompletely melted, their material does not become thoroughly intermixed with the material of the smaller particles. A more thorough melting of the constituents of a flow of large and small particles can be effected if, as the material is heated and fed along a flow path, the larger particles in the mixture are effectively brought into engagement with the heated inner surfaces of the barrel-like body so that these large particles can absorb sufficient heat energy to melt properly.
One type of approach which has been taken in an effort to achieve better mixing and blending of flows of thermoplastics material has been to alter the configurations of augers used to feed the flows Proposed auger improvements have included auger pitch modifications which are intended to progressively increase or otherwise vary the feed pressure of the flow as it moves through the barrel-like body. Although a variety of auger-related improvements have been proposed, incomplete mixing continues to pose a problem.
Another proposed approach has been to position devices such as screens, sieves, or plates having arrays of straight-through holes across a fluid flow path to cause flowing fluid to divide itself into a plurality of individual flows for passage through these devices, whereafter the flows recombine. To the degree that proposals of this type have provided any improvement in mixing, the result, in most instances, has not been satisfactory, especially in the environment of a plastics forming machine where the thermoplastics material to be mixed and blended is in the process of being melted, is in something of a semi-solid state, and is sufficiently viscous to seriously resist ordinary efforts to effect a proper mixing and blending action.
There remains a need for a simple and effective system for properly and thoroughly mixing and blending the constituents of flow of viscous material such as the constituents of a flow of thermoplastics material as they are being melted and caused to flow under pressure through the barrel-like body of a plastics forming machine.